Hybrid electric vehicles achieve high fuel mileage and low vehicle emissions by combining a battery-powered electric motor/generator (MG) with a highly efficient heat engine, typically an internal combustion engine (ICE). By using on-board engine computer controls to vary when the motor/generator or the heat engine, or both, are used, the hybrid motor vehicle can achieve peak efficiency in different driving conditions. A parallel hybrid electric vehicle (in contrast to a series hybrid) uses both the electric motor/generator and the heat engine to provide power to the driving wheels. Most parallel hybrid vehicles do not fix the ratio of power from the electric motor/generator and the heat engine, but rather vary the ratio of power from the electric motor/generator and heat engine depending on which engine or motor or combination thereof has the greatest efficiency in a particular situation. The motor vehicle MG functions as both a motor, delivering torque through some mechanism to the drive wheels, and as a generator, powering the motor vehicle's electrical system. When the MG is functioning as a generator, it may either be powered by torque from the motor vehicle ICE or the wheels of the motor vehicle. The MG also works a starter motor, spinning up the ICE to an engine rotation speed at which combustion is possible and efficient.
The efficiency of a hybrid electric vehicle can be improved further by using a continuously variable transmission (CVT). A continuously variable transmission operates in a similar manner to an ‘automatic’ transmission in that it does not require any clutch or shifter modulation by the driver of the vehicle, as is the case with a ‘manual’ transmission. Unlike traditional ‘automatic’ or ‘manual’ transmissions, however, a CVT does not have fixed gear ratios. Between the maximum and minimum possible gear ratios, a CVT has an infinite number of possible gear ratios. One form of continuously variable transmission works by having a belt turn on two pulleys. One pulley is connected to the input sheave of the transmission, which receives power from the engine of the vehicle. The other pulley is connected to the output sheave of the transmission, which delivers power to the wheels of the vehicle. Each of the pulleys consists of two halves, each having a V-shaped inner surface, along which the belt connecting the two pulleys rides. By varying the distance between the two halves of one of the pulleys, the effective diameter of the pulley is changed owing to the V-shaped surface of the pulley. Thus, the ratio of the transmission can be changed steplessly by altering the spacing between the pulley halves. In addition to having an infinitely variable gear ratio, a CVT also has the advantage that the gear ratio can be changed more smoothly than is possible with either an automatic or a manual transmission.
In a hybrid electric vehicle, regardless of transmission type, it is desirable to be able to cut fuel to the heat engine during deceleration (to save fuel) but to be able to quickly restart the heat engine if and when the driver desires to cause the vehicle to accelerate. One conventional approach to engine restarting in hybrid vehicles is accomplished by using the MG to spin up the ICE to an engine speed at which combustion in the engine is possible and efficient. This approach potentially has problems, however; for example, the battery pack that powers the MG may not have enough charge to start the ICE. Another approach to restarting the ICE is to leave the engine running as the vehicle slows to a stop and use regenerative braking to charge up the MG battery pack. The MG functions as a generator as the vehicle is slowing, taking power from the ICE or the vehicle wheels and acting as a generator in order to charge the MG battery packs. As soon as the vehicle stops, the ICE turns off, and the MG battery packs have enough charge to restart the ICE. This approach also has problems, however. Leaving the engine running as the vehicle slows is not as fuel efficient as shutting off the engine during deceleration for the obvious reason that the ICE is on longer and thus burns more fuel.
Accordingly, it is desirable to provide a CVT hybrid powertrain fueling and engine stop-start control method that is both fuel efficient and smooth. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.